Thank you for your help!
I actually didn't have to install the PW first.
I was able to use the old version and only had to enter the new access data for the database.
The error was caused by a few cache files (*.cache).
I deleted them and the site works!
I still have one question:
How do I get into the backend? /processwire/ does not work.
Could it be that the backend login has been renamed? Can I find out? I have already looked in the database, but found nothing.